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United States Patent |
5,734,679
|
Nomura
|
March 31, 1998
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Voice signal transmission system using spectral parameter and voice
parameter encoding apparatus and decoding apparatus used for the voice
signal transmission system
Abstract
The invention provides a voice signal transmission system which reduces the
amount of increase in calculation while suppressing possible deterioration
of the performance by an expansion in the distribution of a voice
parameter representative of an envelope of a voisce pectrum when input
voice having a plurality of frequency characteristics is treated
simultaneously. Discrimination circuit 5 discriminates, based on a voice
parameter extracted by spectrum parameter extraction circuit 2, another
voice parameter obtained by quantization of the extracted voice parameter
by first quantization circuit 3 and a further voice parameter obtained by
quantization of the extracted audio parameter by second quantization
circuit 4, which one of either third quantization circuit 6 designed
corresponding to first quantization circuit 3 or fourth quantization
circuit 7 designed corresponding to second quantization circuit 4 should
be used. Then, in accordance with a result of the discrimination, third
quantization circuit 6 or fourth quantization circuit 7 quantizes a
quantization error of corresponding first quantization circuit 3 or second
quantization circuit 4 as an error vector. On the reception side, the
error vector is dequantized in accordance with the discrimination.
Inventors:
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Nomura; Toshiyuki (Tokyo, JP)
|
Assignee:
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NEC Corporation (JP)
|
Appl. No.:
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584950 |
Filed:
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January 16, 1996 |
Foreign Application Priority Data
Current U.S. Class: |
375/245; 375/240; 704/222; 704/230 |
Intern'l Class: |
H04B 014/06 |
Field of Search: |
375/240,243,245
395/2.31,2.39,2.3,2.32,2.14,2.18,2.28,2.29
|
References Cited
U.S. Patent Documents
5271089 | Dec., 1993 | Ozawa | 395/2.
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5487128 | Jan., 1996 | Ozawa | 395/2.
|
Other References
Furui, S., "Digital Voice Processing", Tokai University Press Association,
pp. 89-94 (with translation), Version 1, Sep. 25, 1985.
Gersho, A. and Gray, R.M., "Vector Quantization and Signal Compression",
Kluwer Academic Publishers, 1991, pp. 309-316.
|
Primary Examiner: Ghebretinsae; Temesghen
Attorney, Agent or Firm: Ostrolenk, Faber, Gerb & Soffen, LLP
Claims
What is claimed is:
1. A voice parameter encoding apparatus, comprising:
a spectral parameter extraction circuit for calculating a voice parameter
representative of a spectrum envelope of a voice input signal for each
frame of every predetermined fixed interval of time;
a first quantization circuit for quantizing the voice parameter outputted
from said spectrum parameter extraction circuit as a parameter having a
first frequency characteristic and outputting a first quantization vector
and for outputting a first code representative of the first quantization
vector;
a second quantization circuit for quantizing the voice parameter outputted
from said spectrum parameter extraction circuit as a parameter having a
second frequency characteristic and outputting a second quantization
vector and for outputting a second code representative of the second
quantization vector;
a discrimination circuit for receiving the first and second quantization
vectors and the voice parameter outputted from said spectrum parameter
extraction circuit, discriminating and selecting the one of either the
first or second quantization vectors which is nearer to the voice
parameter outputted from said spectrum parameter extraction circuit,
calculating a difference between the selected first or second quantization
vector and the voice parameter outputted from said spectrum parameter
extraction circuit as an error vector, outputting a first code or a second
code representative of the selected first or second quantization vector
together with discrimination information, and outputting, when the first
quantization vector is selected, the calculated error vector to a first
route, but outputting, when the second quantization vector is selected,
the calculated error vector to a second route;
a third quantization circuit for quantizing, when the error vector is
outputted from said discrimination circuit to said first route, the
outputted error vector and outputting a third code corresponding to the
quantization vector obtained by the quantization;
a fourth quantization circuit for quantizing the outputted error vector
when the error vector is outputted from said discrimination circuit to
said second route, and outputting a fourth code corresponding to the
quantization vector obtained by the quantization; and
a transmission circuit for receiving the first or second code outputted
from said discrimination circuit, the discrimination information, and the
third or fourth code outputted from said third or fourth quantization
circuit as inputs thereto and outputting the inputs to a transmission
line.
2. A voice parameter encoding apparatus as claimed in claim 1, wherein said
discrimination circuit refers, upon selection of the one of either the
first or second quantization vectors which is nearer to the voice
parameter outputted from said spectrum parameter extraction circuit, a
weight as a result of discrimination performed in the past.
3. A voice parameter decoding apparatus for decoding a transmission signal
from a voice parameter encoding apparatus which includes a spectral
parameter extraction circuit for calculating a voice parameter
representative of a spectrum envelope of a voice input signal for each
frame of every predetermined fixed interval of time, a first quantization
circuit for quantizing the voice parameter outputted from said spectrum
parameter extraction circuit as a parameter having a first frequency
characteristic and outputting a first quantization vector and for
outputting a first code representative of the first quantization vector, a
second quantization circuit for quantizing the voice parameter outputted
from said spectrum parameter extraction circuit as a parameter having a
second frequency characteristic and outputting a second quantization
vector and for outputting a second code representative of the second
quantization vector, a discrimination circuit for receiving the first and
second quantization vectors and the voice parameter outputted from said
spectrum parameter extraction circuit, discriminating and selecting the
one of either the first or second quantization vectors which is nearer to
the voice parameter outputted from said spectrum parameter extraction
circuit, calculating a difference between the selected first or second
quantization vector and the voice parameter outputted from said spectrum
parameter extraction circuit as an error vector, outputting a first code
or a second code representative of the selected first or second
quantization vector together with discrimination information, and
outputting, when the first quantization vector is selected, the calculated
error vector to a first route, but outputting, when the second
quantization vector is selected, the calculated error vector to a second
route, a third quantization circuit for quantizing, when the error vector
is outputted from said discrimination circuit to said first route, the
outputted error vector and outputting a third code corresponding to the
quantization vector obtained by the quantization, a fourth quantization
circuit for quantizing the outputted error vector when the error vector is
outputted from said discrimination circuit to said second route, and
outputting a fourth code corresponding to the quantization vector obtained
by the quantization, and a transmission circuit for receiving the first or
second code outputted from said discrimination circuit, the discrimination
information, and the third or fourth code outputted from said third or
fourth quantization circuit as inputs thereto and outputting the inputs to
a transmission line as a transmission signal, said voice parameter
decoding apparatus comprising:
a reception circuit for receiving the transmission signal, discriminating
from the discrimination information of the transmission signal whether a
discrimination signal is originated from one of either said first or third
quantization circuit or from one of either said second or fourth
quantization circuit and outputting, when a result of the discrimination
shows that the transmission signal originates from said first or third
quantization circuit, the transmission signal to a third route, but
outputting, when the result of the discrimination shows that the
transmission signal originates from the second or fourth quantization
circuit, the transmission signal to a fourth route;
a first dequantization circuit for dequantizing the first code;
a third dequantization circuit for dequantizing the third code;
a first adder circuit for adding outputs of said first and third
dequantization circuits and outputting a result of the addition to an
output terminal when the transmission signal is outputted to said third
route; and
a second dequantization circuit for dequantizing the second code;
a fourth dequantization circuit for dequantizing the fourth code;
a second adder circuit for adding outputs of said second and fourth
dequantization circuits and outputting a result of the addition to said
output terminal when the transmission signal is outputted to said fourth
route.
4. A voice signal transmission system, comprising a voice parameter
encoding apparatus as claimed in claim 1, a voice parameter decoding
apparatus as claimed in claim 3, and a transmission line for
interconnecting between a transmission circuit of said voice parameter
encoding apparatus as claimed in claim 1 and a reception circuit of said
voice parameter decoding apparatus as claimed in claim 3.
5. A voice signal transmission system, comprising a voice parameter
encoding apparatus as claimed in claim 2, a voice parameter decoding
apparatus as claimed in claim 3, and a transmission line for
interconnecting between a transmission circuit of said voice parameter
encoding apparatus as claimed in claim 2 and a reception circuit of said
voice parameter decoding apparatus as claimed in claim 3.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a voice signal transmission system which encodes
a voice signal using a vector quantization circuit, transmits the coded
audio signal and decodes the coded voice signal effectively at the
receiver side.
2. Description of Related Art
Vector quantization is known as an effective method of transmitting and
storing voice data. Vector quantization is a method for selecting the code
the vector whose distance from an input vector is the shortest from a code
book having a plurality of code vectors designed in advance. By
transmitting and storing the selected code (number) representative of the
code vector, a voice input signal can be transmitted and stored
effectively. Details of the vector quantization and multistage vector
quantization are disclosed, in A. Gersho et al., "Vector Quantization and
Signal Compression", Kluwer Academic Publishers.
When a voice parameter encoding apparatus is realized using the vector
quantization described above, and if input voice having a plurality of
frequency characteristics is treated by the same encoding apparatus, the
distribution of a voice parameter which represents an envelope of a voice
spectrum will expand, resulting in deterioration of the performance of the
voice parameter encoding apparatus. As a countermeasure against the
deterioration of the performance, a method wherein the number of
quantization bits of an audio parameter which represents an envelope of a
voice spectrum is increased and another method wherein a quantization
circuit is prepared for each frequency characteristic to detect an
available optimum quantization value are adaptable.
Operation of a voice signal transmission system which encodes a voice
parameter using the latter method is described below with reference to
FIG. 1. For simplified description, it is assumed that the input voice
data has two different frequency characteristics and a quantization
circuit is designed for each of the characteristics respectively. Here, it
is assumed that the two frequency characteristics of input voice are
frequency characteristic (hereinafter referred to as FLAT characteristic)
in which the voice band is limited to a normal voice band and another
frequency characteristic (hereinafter referred to as IRS characteristic)
which is emphasized in the high frequency region.
The spectrum parameter extraction circuit 32 calculates a parameter
representative of a spectrum envelope of input voice data inputted through
input terminal 31 for a frame after every fixed interval of time, and
outputs the parameter as an input vector to first quantization circuit 33
and second quantization circuit 34. As the parameter representative of a
spectrum envelope, a known parameter called line spectrum pair (LSP) is
available. A method of analyzing a line spectrum pair is disclosed in
Furui, "Digital voice Processing", the Publishing Society of Tokai
University.
The first quantization circuit 33 is designed for the FLAT characteristic
while second quantization circuit 34 is designed for the IRS
characteristic. The first quantization circuit 33 quantizes the input
vector using the vector quantization described above and outputs the
quantization vector to discrimination circuit 35. Further, first
quantization circuit 33 outputs a code corresponding to the quantization
vector to discrimination circuit 35.
Similarly, second quantization circuit 34 quantizes the input vector using
the vector quantization described above and outputs the quantization
vector to discrimination circuit 35. Further, second quantization circuit
34 outputs a code corresponding to the quantization vector to
discrimination circuit 35.
The discrimination circuit 35 discriminates the characteristic of an input
vector, either the FLAT characteristic or the IRS characteristic, based on
the quantization vectors of first quantization circuit 33, second
quantization circuit 34 and the input vector. Then, discrimination circuit
35 outputs a code of the input voice corresponding to the frequency
characteristic and discrimination information representative of a result
of the discrimination through transmission circuit 36.
In the decoding apparatus, reception circuit 37 receives the code and the
discrimination information transmitted thereto from transmission circuit
36 and is selectively connected to first dequantization circuit 38 or
second dequantization circuit 39 in response to the discrimination
information so that the selectively connected dequantization circuit may
perform dequantization of the code to produce a dequantization vector
corresponding to the code. The dequantization code is outputted from
output terminal 40.
However, since all of the prior art apparatus described above require
comparison processing with a large number of code vectors, the amount of
required calculation is very great. Further, even if multistage vector
quantization which involves a reduced amount of calculation is used, real
time processing is still difficult.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a voice signal
transmission system which reduces the quantity of calculation by
suppressing possible deterioration of performance due to expansion in the
distribution of a voice parameter representative of an envelope of a voice
spectrum when input voice data having a plurality of frequency
characteristics is treated simultaneously and a voice parameter encoding
apparatus and decoding apparatus for use with the voice signal
transmission system.
In order to attain the object described above, according to an aspect of
the present invention, there is provided a voice parameter encoding
apparatus, comprising a spectral parameter extraction circuit for
calculating a voice parameter representative of a spectrum envelope of a
voice input signal for each frame of every predetermined fixed interval of
time, a first quantization circuit for quantizing the voice parameter
outputted from the spectrum parameter extraction circuit as a parameter
having a first frequency characteristic and outputting a first
quantization vector and for outputting a first code representative of the
first quantization vector, a second quantization circuit for quantizing
the voice parameter outputted from the spectrum parameter extraction
circuit as a parameter having a second frequency characteristic and
outputting a second quantization vector and for outputting a second code
representative of the second quantization vector, a discrimination circuit
for receiving the first and second quantization vectors and the voice
parameter outputted from the spectrum parameter extraction circuit,
discriminating and selecting the one of either the first or second
quantization vectors which is nearer to the audio parameter outputted from
the spectrum parameter extraction circuit, calculating a difference
between the selected first or second quantization vector and the voice
parameter outputted from the spectrum parameter extraction circuit as an
error vector, outputting a first code or a second code representative of
the selected first or second quantization vector together with
discrimination information, and outputting, when the first quantization
vector is selected, the calculated error vector to a first route, but
outputting, when the second quantization vector is selected, the
calculated error vector to a second route, a third quantization circuit
for quantizing, when the error vector is outputted from the discrimination
circuit to the first route, the outputted error vector and outputting a
third code corresponding to the quantization vector obtained by the
quantization, a fourth quantization circuit for quantizing the outputted
error vector when the error vector is outputted from the discrimination
circuit to the second route, and outputting a fourth code corresponding to
the quantization, vector obtained by the quantization, and a transmission
circuit for receiving the first or second code outputted from the
discrimination circuit, the discrimination information, and the third or
fourth code outputted from the third or fourth quantization circuit as
inputs thereto and outputting the inputs to a transmission line.
Preferably, the discrimination circuit refers, upon selection of the one of
either the first or second quantization vectors which is nearer to the
audio parameter outputted from the spectrum parameter extraction circuit,
a weight as a result of discrimination performed in the past.
According to another aspect of the present invention, there is provided a
voice parameter decoding apparatus for decoding a transmission signal from
a voice parameter encoding apparatus which includes a spectral parameter
extraction circuit for calculating a voice parameter representative of a
spectrum envelope of a voice input signal for each frame of every
predetermined fixed interval of time, a first quantization circuit for
quantizing the voice parameter outputted from the spectrum parameter
extraction circuit as a parameter having a first frequency characteristic
and outputting a first quantization vector and for outputting a first code
representative of the first quantization vector, a second quantization
circuit for quantizing the voice parameter outputted from the spectrum
parameter extraction circuit as a parameter having a second frequency
characteristic and outputting a second quantization vector and for
outputting a second code representative of the second quantization vector,
a discrimination circuit for receiving the first and second quantization
vectors and the audio parameter outputted from the spectrum parameter
extraction circuit, discriminating and selecting the one of either the
first or second quantization vectors which is nearer to the voice
parameter outputted from the spectrum parameter extraction circuit,
calculating a difference between the selected first or second quantization
vector and the voice parameter outputted from the spectrum parameter
extraction circuit as an error vector, outputting a first code or a second
code representative of the selected first or second quantization vector
together with discrimination information, and outputting, when the first
quantization vector is selected, the calculated error vector to a first
route, but outputting, when the second quantization vector is selected,
the calculated error vector to a second route, a third quantization
circuit for quantizing the outputted error vector when the error vector is
outputted from the discrimination circuit to the first route, and
outputting a third code corresponding to the quantization vector obtained
by the quantization, a fourth quantization circuit for quantizing, when
the error vector is outputted from the discrimination circuit to the
second route, the outputted error vector and outputting a fourth code
corresponding to the quantization vector obtained by the quantization, and
a transmission circuit for receiving the first or second code outputted
from the discrimination circuit, the discrimination information, and the
third or fourth code outputted from the third or fourth quantization
circuit as inputs thereto and outputting the inputs to a transmission
line, the voice parameter decoding apparatus comprising a reception
circuit for receiving the transmission signal, discriminating from the
discrimination information of the transmission signal whether the
discrimination signal is originated from the first or third quantization
circuit or from the second or fourth quantization circuit and outputting,
when a result of the discrimination shows that the transmission signal
originates from the first or third quantization circuit, the transmission
signal to a third route, but outputting, when the result of the
discrimination shows that the transmission signal originates from the
second or fourth quantization circuit, the transmission signal to a fourth
route, a first dequantization circuit for dequantizing the first code and
a third dequantization circuit for dequantizing the third code as well as
a first adder circuit for adding outputs of the first and third
dequantization circuits and outputting a result of the addition to an
output terminal when the transmission signal is outputted to the third
route, and a second dequantization circuit for dequantizing the second
code and a fourth dequantization circuit for dequantizing the fourth code
as well as a second adder circuit for adding outputs of the second and
fourth dequantization circuits and outputting a result of the addition to
the output terminal when the transmission signal is outputted to the
fourth route.
According to a further aspect of the present invention, there is provided a
voice signal transmission system, comprising the voice parameter encoding
apparatus described above, the voice parameter decoding apparatus
described above, and a transmission line for interconnecting between the
transmission circuit of the audio parameter encoding apparatus and the
reception circuit of the voice parameter decoding apparatus.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing a system construction of an example of a
conventional audio signal transmission system;
FIG. 2 is a block diagram showing a system construction of a first
embodiment of the voice signal transmission system of the present
invention; and
FIG. 3 is a block diagram showing a system construction of a second
embodiment of the voice signal transmission system of the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiments of the present invention are described below with reference to
the drawings. FIG. 2 is a block diagram showing a system construction of a
first embodiment of the voice signal transmission system of the present
invention. In the present embodiment, for practical and simplified
description, it is assumed that input voice has two different frequency
characteristics. Here, it is assumed that the two frequency
characteristics of the input voice are a frequency characteristic
(hereinafter referred to as FLAT characteristic) in which the voice band
is limited normaly and another frequency characteristic (hereinafter
referred to as IRS characteristic) in which the voice is emphasized in a
high frequency region.
The spectrum parameter extraction circuit 2 calculates a parameter
representative of a spectrum envelope of input voice inputted though input
terminal 1 for a frame after every fixed number of frames, and outputs the
calculated parameter as an input vector to first quantization circuit 3,
second quantization circuit 4 and discrimination circuit 5. As the
parameter representative of a spectrum envelope, a known parameter called
line spectrum pair is used. Naturally, the parameter representative of a
spectrum envelope is not limited to the line spectrum pair.
The first quantization circuit 3 is designed for the FLAT characteristic
described above. The first quantization circuit 3 quantizes the input
vector from spectrum parameter extraction circuit 2 and outputs a
quantization vector obtained as a result of the vector quantization to
discrimination circuit 5. Further, first quantization circuit 3 outputs a
code corresponding to the quantization vector to discrimination circuit 5.
The second quantization circuit 4 is designed for the IRS characteristic
described above. The second quantization circuit 4 quantizes the input
vector from spectrum parameter extraction circuit 2 and outputs the
quantization vector obtained as a result of the vector quantization to
discrimination circuit 5. Further, second quantization circuit 4 outputs a
code corresponding to the quantization vector to discrimination circuit 5.
The discrimination circuit 5 discriminates to which frequency
characteristic of the FLAT characteristic or the IRS characteristic an
input voice belongs, based on the quantization vector of first
quantization circuit 3, the quantization vector of second quantization
circuit 4 and the input vector. Then, discrimination circuit 5 subtracts
the quantization vector of the discriminated characteristic from the input
vector to calculate an error vector. When the discrimination result is the
FLAT characteristic, discrimination circuit 5 outputs the calculated error
vector to third quantization circuit 6 and outputs the code corresponding
to the quantization vector which was based on the calculation of the error
vector and discrimination information representative of the discrimination
result to transmission circuit 8. When the discrimination result is the
IRS characteristic, discrimination circuit 5 outputs the calculated error
vector to fourth quantization circuit 7 and outputs the code corresponding
to the quantization vector which was based on the calculation of the error
vector and discrimination information representative of the discrimination
result to transmission circuit 8.
The third quantization circuit 6 is designed for the FLAT characteristic so
that it may quantize the error vector of first quantization circuit 3. The
third quantization circuit 6 quantizes the inputted error vector based on
the discrimination result outputted from discrimination circuit 5, and
outputs a code corresponding to the quantized error vector to transmission
circuit 8.
The fourth quantization circuit 7 is designed for the IRS characteristic in
order to quantize the error vector of second quantization circuit 4. The
fourth quantization circuit 7 quantizes the inputted error vector based on
the discrimination result outputted from discrimination circuit 5 and
outputs a code corresponding to the quantized error vector to transmission
circuit 8.
The transmission circuit 8 transmits the code and the discrimination
information representative of the discrimination result inputted from
discrimination circuit 5 as well as a code obtained from third
quantization circuit 6 or fourth quantization circuit 7 to a decoding
apparatus.
The reception circuit 9 in the decoding apparatus receives the code and the
discrimination information transmitted from transmission circuit 8. When
the received discrimination information represents the FLAT
characteristic, reception circuit 9 outputs the received code to first
dequantization circuit 10 and third dequantization circuit 11. When the
received discrimination information represents the IRS characteristic,
reception circuit 9 outputs the received code to second dequantization
circuit 12 and fourth dequantization circuit 13.
The first dequantization circuit 10 performs dequantization corresponding
to the quantization of first quantization circuit 3, and third
dequantization circuit 11 performs dequantization corresponding to the
quantization of third quantization circuit 6. Meanwhile, second
dequantization circuit 12 performs dequantization corresponding to the
quantization of second quantization circuit 4, and fourth dequantization
circuit 13 performs dequantization corresponding to the quantization of
fourth quantization circuit 7.
The first addition circuit 14 adds the quantization vector from first
dequantization circuit 10 and the quantization vector from third
dequantization circuit 11 and outputs a result of the addition to output
terminal 16. The second addition circuit 15 adds the quantization vector
from second dequantization circuit 12 and the quantization vector from
fourth dequantization circuit 13 and outputs a result of the addition to
output terminal 16.
While the embodiment described above is applied to the case wherein input
voice has two different frequency characteristics, a method of increasing
the number P, the number of frequency characteristics, can be analogized
readily. Further, when the number P, the number of frequency
characteristics, is increased, the number K (K<P), the number of potential
frequency characteristics, shall be quantized by discrimination circuit 5
to determine a corresponding frequency characteristic and a corresponding
code based on a final result of the quantization.
The second embodiment of the present invention is described below with
reference to FIG. 3. The frequency characteristic of the input voice data
does not vary with data unit of frame for which processing is performed,
but relies upon the entire input voice data to the audio parameter
encoding apparatus. Therefore, when the discrimination circuit
discriminates to which one of either the FLAT characteristic or the IRS
characteristic an inputted voice belongs, the deterioration of the
performance of the voice parameter encoder caused by an error in
discrimination can be further reduced by discriminating the present frames
on the basis of weighting the results of their past discrimination
respectively. For simplified description of the second embodiment,
operation of discrimination circuit 5 using a result or results of
discrimination in the past is described herein after. The operations of
the other components are the same as those of the first embodiment shown
in FIG. 2.
The discrimination circuit 25 discriminates to which one of either the FLAT
characteristic or the IRS characteristic the inputted voice data belongs,
based on a result or results of past discrimination obtained from delay
circuit 25a, a quantization vector of first quantization circuit 3,
another quantization vector of second quantization circuit 4 and an input
vector. Then, discrimination circuit 25 subtracts the quantization vector
of the discriminated characteristic from the input vector to obtain an
error vector and outputs the error vector to third quantization circuit 6
or fourth quantization circuit 7 in response to the result of
discrimination. Further, discrimination circuit 25 outputs a corresponding
code and discrimination information representative of the result of
discrimination to transmission circuit 8.
As a method for weighting an evaluated value at present with a result or
results of past discrimination, for example, the following method may be
used. For the evaluated value, a square of distance between the
quantization vectors obtained from the first and second quantization
circuits and the input vector is used.
(1) The weighting coefficient to a quantization vector for the frequency
characteristic which has the same result of past discrimination is set as
a predetermined value W (W<1, for example, 0.8), and the weighting
coefficient to a quantization vector for the other frequency
characteristic having no discrimination result is set as 1.0.
(2) When the same result of discrimination successively occurs, weighting
coefficient W(x) is varied with the number (x) of the successive frames.
For example, weighting coefficients W(x) are set to W(0)=1.0, W(1)=0.9,
W(2)=0.8, . . . , and W(5)=0.5. In the present example, when the
repetition number x of the same discrimination result is greater than 5, x
is set to x=5. By discriminating the frequency characteristic of the input
voice using a result of past discrimination in accordance with the method
described above, the discrimination value can be stabilized in successive
frames.
As described above, according to the present invention, since it is
discriminated to which frequency characteristics an input vector belongs
and limits the operation of quantization circuit only for the quantization
circuits which are provided for the discriminated frequency
characteristic, the amount of calculation can be reduced, and
deterioration-in performance can be prevented.
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